The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties therefor.
The invention relates in general to optical measurement of pressure and in particular to optical measurement of pressure based on the reflectance of the pressurized material.
Some known methods use ruby fluorescence to measure the temporal profile of pressure as a shock traverses a material. Two groups have demonstrated ruby fluorescence as a dynamic pressure gauge (See P. D. Horn, and Y. M. Gupta, Phys. Rev. 39, 973 (1989) and G. I. Pangilinan, M. R. Baer, J. Namkung, P. Chambers, and T. P. Russell, Appl. Phys. Lett. 77, 684 (2000)).
It is been shown that a sufficient signal can be obtained from the ruby fluorescence. The ruby fluorescence has a property that is related to pressure and can be calibrated at static conditions. The optical character of the ruby sensor is advantageous because it is insulated from strong electromotive forces inherent in triggering most shock applications. These electromotive forces render piezoresistive and piezoelectric gauges, both attractive in measuring shock arrival time and peak amplitude, ineffective.
FIG. 1 shows a known apparatus for measuring the temporal profile of pressure in a shock wave using time-resolved ruby fluorescence spectra. A 2-watt laser 10 operating at 532 nm is used to excite the ruby fluorescence. At this wavelength, the excitation beam is transmitted through a dichroic beam splitter 12 and is focused by a lens 14 into an optical fiber 16. The ruby crystal 18 is epoxied to the end of the fiber 16 and is appropriately placed where the pressure is to be measured. Without lost of generality, the pressure measured is pressure underwater. The ruby fluorescence (at 693 and 694 nm) is collected by fiber 16, reflected by the beam splitter 12 and focused by a lens 19 to a second optical fiber 20. The signal is relayed to a spectrometer 22 by the second optical fiber 20. The spectrometer 22 disperses the signal as a function of wavelength. The collected signal from the spectrometer 22 is further dispersed in time orthogonal to the wavelength dispersion by a streak camera 24 which outputs intensity as a function of two dimensions: wavelength and time.
The streak rate of the camera 24 can be set to provide fast 20 ns pressure data points for up to 0.8 microseconds total. Slower snapshots are readily attainable all with a maximum of 40 data points (nominally twenty data points are obtained from a single measurement). A two dimensional charge-coupled device (CCD) 26 is used to collect and digitize the intensity as a function of wavelength and time. The laser 10, streak camera 24, and CCD 26 are synchronized with the arrival of the shock at the ruby sensor 18 by a delay generator 28.
Examples of the ruby spectra before and during the shock passage are shown in FIG. 2. The shifts of the positions of the ruby fluorescence are used to infer the pressure. The fluorescence yields the pressures as the shock crosses the ruby sensor 18. There are roughly twenty time intervals where pressure is obtained in such an apparatus.
The invention comprises an apparatus for measuring pressure in a medium that includes a laser for emitting linearly polarized light, a polarizing beam splitter that reflects the linearly polarized light from the laser, a first lens that receives and focuses the linearly polarized light from the polarizing beam splitter, and an optical fiber. The first end of the optical fiber receives linearly polarized light from the first lens and the second end of the optical fiber comprises a tip, disposed in the medium, that receives reflected light from the medium. The reflected light is transmitted back through the optical fiber and the first lens to the polarizing beam splitter. The polarizing beam splitter transmits the reflected light that has polarization orthogonal to the linearly polarized light emitted by the laser, but not transmitting the reflected light that has polarization substantially parallel to the linearly polarized light emitted by the laser. The invention also comprises a second lens that receives the reflected light that has polarization orthogonal to the linearly polarized light emitted by the laser. A photodiode that receives the orthogonal reflected light from the second lens. An oscilloscope is connected to the photodiode and a delay generator is connected to the photodiode, the oscilloscope and the laser.